Sales at fast casual chain Chipotle Mexican Grill continue to tumble as the company's struggle with a bout of foodborne illness issues at locations across the country deepens.

The company, on Wednesday, reported a double dose of bad news. Sales at restaurants open at least a year slid 30% in December and fell 14.6% during the fourth quarter. The company had previously predicted an 11% decline for the period. Chipotle also said it has been served with a federal grand jury ...